This invention relates to electric arc furnaces and more particularly to controls for positioning the electrodes of electric arc furnaces.
Electric arc furnaces are commonly employed for melting or treating metallic furnace charges. The heat required for such fusion or treatment is provided by the arcs maintained between the electrodes and the contents of the furnace. It will be appreciated that the position of the electrodes relative to the furnace charge affects the voltage and current parameters. As a result, arc furnace electrodes must be positioned vertically at proper elevations relative to the furnace charge in order to maintain furnace electrical conditions within desired limits.
One prior art method of controlling the position of arc furnace electrodes was to employ impedances for generating voltage signals functionally related to arc current and voltage. However, the impedance method of electrode control does not provide proper management of the power supplied to the furnace during operations such as, for example, scrap melting and smelting. One problem is that most measurements of secondary electrode current are inaccurate owing to uneven distribution of current in the secondary conductors and to the fact that only a portion of the current is usually sampled and the final value is the result of arbitrary proportioning. Further, inaccuracies are introduced because the currents in the three phase electrodes are interrelated so that a change in value in one electrode affects the current levels in the other two. Additionally, current and/or voltage signal levels change with changes in transformer tap settings, making it necessary to accommodate different levels of signal strength. Furthermore, the signals are generally rectified which eliminates the phase displacement between voltage and current that is necessary in order to obtain true impedance values. The secondary voltage and current signals also do not provide an accurate measure of the individual phase impedances because there is no common neutral since the secondary circuit within the furnace bath represents a delta load with the arcs acting as switches to transfer current from one phase to another.